FIELD OF THE INVENTION
The present invention generally relates to a method for making a building board, such as a floorboard, which board is intended to be mechanically joined to similar building boards and which board comprises a board body as well as, for the mechanical joining, a metal strip which is mechanically connected to, and projects from, the board body and which is formed with a locking element intended to engage with a complementary locking groove of an adjoining building board.
More specifically, the invention relates to an improved technique for the mechanical connection between the metal strip and the body.
BACKGROUND, FEATURES AND ADVANTAGES OF THE INVENTION
A building board, for example a floorboard, provided with a projecting metal strip formed with a locking element for mechanical joining is described in WO 94/26999. The content of that document shall be considered to be part of the present description, and provides a more detailed description of how such building boards can be designed and joined together. The background, features and advantages of the invention will be described specifically for this known type of floorboard, but it should be emphasised that the invention is useful for making building board types other than floorboards, such as wall panels and roof slabs. All references to the term “floorboard” should therefore be considered to apply to building boards in general.
WO 94/26999 thus discloses a system for mechanical joining of floorboards. A first mechanical connection provides mutual vertical locking of the joint edges and may be in the form of a tongue-and-groove joint along the joint. A second mechanical connection provides mutual horizontal locking of the boards in a direction at right angles to the joint edges of the boards.
In order to illustrate the situation upon which the present invention is based, reference is now made to FIG. 1, which shows in section a joint between two identical mechanically joined floorboards 2. The method according to the invention is useful for making such floorboards. The design and the function of the floorboards 2 substantially correspond to what is known from WO 94/26999. However, there are certain differences compared to the prior art with respect to the geometrical shapes of a gripping stud and a locking element.
Each board 2 has a top side 4 and underside 6 and, for illustration purposes, can be assumed to be made of a board body S of laminated fibreboard, plastic composite, wood or the like. The thickness of the body S can, for example, be 7 mm. To enable a mechanical connection, opposite joint edges 8 of the boards 2 are formed with an integrated metal strip 10 mounted at the factory, as well as a locking groove 16. The strip 10 is preferably made of sheet aluminium and extends horizontally from the underside 6 of the board 2 in the direction of the second floorboard and runs continuously throughout the entire length of the joint. However, the strip 10 can be divided into smaller parts, which cover the main portion of the length of the joint.
In order to achieve the required joint tolerances as well as simple laying, the strip 10 is integrally formed with the board, i.e. it is mounted at the factory and should specifically not be mounted in connection with laying. As a non-restrictive example, the strip 10 may have a width of about 30 mm and a thickness of about 0.6 mm. Metal sheet materials other than aluminium could also be used.
Along its one side edge, the strip 10 is formed with a locking element 12, bent from the sheet material, which exhibits an active locking surface 14 having a height of e.g. 1.0 mm. In the joined state, the locking element 12 is received in a locking groove 16, formed in the underside 6 of the second board and extending parallel to and spaced from the joint edge 8. The locking element 12 and the locking groove 16 together form the above-mentioned second mechanical connection, locking the boards 2 to each other in the direction designated D2. More specifically, the locking surface 14 of the locking element 12 serves as a stop with respect to the surface 18 of the locking groove 16 closest to the joint edges 8.
When the boards 2 are joined together according to FIG. 1, they can occupy a relative position in the direction D2 where a small play Δ, as small as 0.01 mm, exists between the locking surface 14 and the locking groove 16. This play makes it possible to displace the boards 2 in the direction of the joint without the use of tools. This displaceability facilitates the laying and enables joining together the short sides by snap action. Reference is made to WO 94/26999 for a more detailed description of the function and advantages of this construction.
The strip 10 is mounted in a tolerance-equalising groove in the underside 6 of the board 2. In this embodiment, the width of the equalising groove is approximately equal to half the width of the strip 10, i.e. about 15 mm. The functioning of and different ways of forming the equalising groove are described in detail in WO 94/26999 and, consequently, need not be repeated here.
The strip 10 is mechanically fitted to the body S in the following manner. A groove 20 is provided in the underside 6 of the body S at a distance from a recess 22 adjacent to the joint edge 8. The groove 20 may be formed either as a continuous groove extending throughout the entire length of the body S, or as a number of separate grooves. Together with the recess 22, this groove 20 defines a dove-tail gripping stud 24 of the body S. In its fastened state in FIG. 1, the strip 10 exhibits a number of punched and bent tongues 26 as well as one or more lips 28, which are bent round opposite sides of the gripping stud 24. The term “gripping element” will be used in the following as a general term for tongues, lips and corresponding components of the strip which are formed from the sheet material and bent round the gripping stud 24 of the body S.
SUMMARY OF THE INVENTION
A main object of the invention is to provide a technique for improving building boards of the type mentioned above.
A particular object of the invention is to provide a technique for improving the mechanical fastening of the strip to the body.
It is also an object of the invention to provide a technique for improving a manufacturing method for building boards of the type mentioned above.
For achieving these and other objects, according to the invention a method is provided for making a building board having the features recited in the appended claims.
Thus, the invention provides a method for making a building board, which exhibits a board body formed with a gripping stud, and a metal strip extending from the body, from which are formed gripping elements which are bent round the gripping stud for mechanical fastening of the strip to the body, as well as a locking element for enabling mechanical joining of the board to similar boards. The method is characterised by preforming the gripping elements of the strip prior to bending them round the gripping stud, and subsequently bending the preformed gripping elements round the gripping stud, the preforming being such that, as a result of the bending, the preformed gripping elements strike against the gripping stud and thereby undergo a deformation in the opposite direction to the preforming in a final stage of the bending.
The preforming which occurs during said final stage of the bending preferably results in a biasing of the gripping elements of the strip against the gripping stud.
The preforming as well as the bending of the gripping elements are preferably carried out by means of punching means operating essentially at right angles to a principal plane of the strip/building board, and, in a particularly preferred embodiment, such punching means are arranged in one and the same punching tool so that they are stationary in relation to each other during the preforming and the bending.
For achieving good fastening, and for eliminating tolerance problems, the gripping elements are preferably preformed to such an extent that said deformation which arises during the final stage of the bending consists of a permanent reverse bending of the gripping elements as well as a resilient return of the gripping elements. In this connection, the reverse bending can compensate for tolerances with respect to, for example, the position of the gripping stud in relation to the board body or in relation to bending punching means, while a biasing force which is achieved by the resilient return can be kept essentially constant.
As is already known per se from the aforementioned WO 94/26999, the gripping stud is preferably provided with undercut gripping edges round which bending is carried out. A particularly strong gripping stud, suitable for the biasing technique according to the invention, can be obtained if the gripping edges of the gripping stud are formed with first non-undercut gripping edge parts closest to the strip and second undercut gripping edge parts adjacent thereto. During the bending, the non-undercut gripping edges achieve a reinforcement of the gripping stud, while the preformed gripping elements are biased essentially only against the undercut gripping edge parts.
Undercut gripping edges of the gripping stud preferably exhibit an undercutting angle of 10°-45° in relation to the normal to a principal plane of the building board, and the gripping elements are preferably preformed with a preforming angle of 15°-90° in relation to a principal plane of the strip. However, for achieving the biasing, the undercutting angle should be smaller than the preforming angle, and preferably so much smaller that, even in the case of deviations due to tolerance in the position of the gripping stud, a permanent reverse bending of the gripping elements as well as a resilient return thereof are always obtained. A return angle in the order of 45° has been found suitable.
In a preferred embodiment, the preforming is achieved by bending each gripping element through a predetermined pre-bending angle at a first point spaced from a free end of the gripping element, and the bending is achieved by bending the gripping element thus preformed at a second point, which is located farther away from said free end than the first point. Specifically, the pre-bending can be performed against a punch die separate from the gripping stud, while the bending is not carried out until the strip has been positioned against the gripping stud, which in this connection serves as a punch die.
These and other embodiments of the invention will appear from the appended claims and the following description of preferred embodiments.
By the biasing technique according to the invention with preforming and bending, several advantages are achieved from a manufacturing as well as a product point of view:
1. The board body, which is typically made of wood or a wood-based material, or of plastic, may change its dimensions in connection with variations in moisture and temperature, while the metal strip is temperature-sensitive only. Such dimensional changes of the body and/or the metal strip may have a negative impact on the mechanical connection between the body and the strip, and may specifically result in undesired joint gaps between the boards as well as poor strength.
A first advantage of the invention is that it ensures that such dimensional changes of the body and/or the strip do not impair the mechanical connection, since, according to the invention, the mechanical connection between the strip and the board body can be biased and, consequently, can automatically and continuously adjust to every dimensional change of these two components. In this way, it is ensured that the strip is always firmly and securely connected to the: board body, so that the relative position of these two components remains correct and unchanged. By the invention, strips which are loose and can be displaced relative to the board body are thus avoided and, consequently, undesired joint gaps and poor strength due to loosely attached strips are eliminated.
2. In addition to the above-mentioned environmentally-caused dimensional variations of the finished building board, a variation can also occur in the position of the gripping stud in relation to the board body. This positional variation is due to tolerances in the manufacturing of the gripping stud, especially if its gripping edges are formed by milling. As a result of these tolerances, the position of the gripping edges in relation to the joint edge of the body may vary somewhat (e.g. in the order of ±0.05 mm) from one building board to another, If the strip is positioned-in relation to the gripping stud at the time of manufacturing, this positional variation of the gripping edges may result in the strip being positioned incorrectly.
A second advantage of the invention is that the preforming in combination with reverse bending and biasing compensates for the above-mentioned positional variation of the gripping edge, since an “incorrect” position of the gripping edges can be compensated for by the fact that the gripping elements of the strip can be caused always to strike against the gripping edges during bending and be reverse bent to different extents, depending upon the position of the corresponding gripping edge.
Generally, in manufacturing, it is desirable to be able to operate within the largest possible tolerances, since this reduces set-up and take-down times, checks, and tool grinding in the present case, a suitably designed preforming can handle tolerances of e.g. 0.15 mm.
3. A third advantage provided by the invention is that, by virtue of being preformed, the gripping elements of the strip can always be moved to the correct position in relation to the bending punches and still strike against and be reverse bent by the gripping stud which is positioned within a certain manufacturing tolerance in relation to the bending punches. This advantage means that even if the relative position between the gripping edges of the body and the bending punches should vary somewhat between different punching operations, this does not have a negative effect on the quality of the mechanical connection between the strip and the board body.
4. A further advantage achieved by the invention is that the biasing force applied to the gripping stud by the bent gripping element of the mechanically attached strip is essentially independent of both the punching force which is applied by means of the bending punches and the length of stroke of the bending punches. The advantage of this is that (i) the bending punches and (ii) other punches required for making the floorboard (such as pre-bending punches, punching machines, etc.) can be mounted in one and the same punching tool, which during manufacturing moves to-and-fro with a length of stroke common to all punches and a common pressing power. Specifically, this makes it possible to allow the bending punches, when they are moving in the direction of the strip, to continue a distance past the point in the punching motion at which the fastening of the strip to the board body is completed, enabling the other punches to complete their punching function during a final, inactive motion of the bending punches.
The biasing force can be controlled with the aid of parameters of the metal strip (sheet thickness, alloy, etc.), as well as with the aid of the position, angle, and length of the preformed gripping elements in relation to the gripping edges and the undercut of the same, and with the aid of the relative position of the gripping edges and the bending punches.
5. The forming of the locking element of the strip is preferably carried out by means of punches operating essentially at right angles to the principal plane of the floorboard, and, as mentioned above, it is an advantage it all punching operations can be carried out with one and the same punching tool. Consequently, it is desirable that the fastening of the strip can also be carried out by means of punches operating at right angles to the principal plane of the floorboard. A further advantage of the invention is that the preforming makes this possible, since the preforming means that the punching equipment need not include bending punches operating from the side for fastening the strip to the gripping stud.
Another advantage of bending punches operating at right angles is that the compression pressure on the same can be optionally very high with no risk of the gripping stud breaking, while the fastening force against the gripping stud can be exactly controlled by the deformation of the strip. Whereas, if the bending punches had been operating towards the edges of the gripping stud, whose position varies in such a punching direction because of the aforementioned manufacturing tolerances, the pressure on the gripping stud would have varied considerably, with the risk of the gripping stud breaking or the fastening of the strip being loose.
6. A further advantage of the invention is that the preforming makes it possible to reduce the thickness of the board body and, consequently, of the finished building board, by virtue of the fact that the height of the gripping stud can be reduced since the gripping elements of the strip, which are to be bent round the gripping stud, are preformed when the bending is carried out.